As is known, many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature processed) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of such a package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing a web of laminated packaging material. The packaging material has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of oxygen-barrier material defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material eventually defining the inner face of the package contacting the food product.
As is known, such packages are made on fully automatic packaging machines, on which a continuous tube is formed from the packaging material supplied in web form.
More specifically, the web of packaging material is sterilized and then fed to a forming unit on which it is folded and sealed longitudinally to form a vertical tube. The tube is filled with the sterilized or sterile-processed food product, and is sealed by pairs of jaws and subsequently cut at equally spaced cross sections to form pillow packs, which are then folded mechanically to form the finished, e.g. substantially parallelepiped-shaped, packages.
Upstream from the forming unit, the web of packaging material may be fed through a processing unit for performing various auxiliary operations, which, when producing packages with opening devices, such as pull-off tabs, screw or hinged caps, may comprise, for example, perforation of a number of through openings or holes at predetermined points on the web, and one or more operations to fix the opening devices over the holes.
The most commonly used opening devices comprise a patch defined by a small sheet of heat-seal plastic material, and which is heat sealed over a respective hole on the side of the web eventually forming the inside of the package; and a pull-off tab applied to the opposite side of the packaging material and heat sealed to the patch. The tab and patch adhere to each other, so that, when the tab is pulled off, the portion of the patch heat sealed to it is also removed to uncover the hole.
To close the package once the tab is pulled off, the portion of the packaging material surrounding the tab is normally fitted with a frame element normally made of plastic material and supporting a removable, e.g. screw or hinged, cap for closing the respective hole.
Alternatively, closable opening devices are also known to be applied by injecting plastic material directly onto the holes in the web, as described, for example, in Patent WO 98/18609.
On known machines, the web of packaging material is fed in steps through the processing unit by a feed system comprising feed rollers controlled by a servomotor in turn controlled in response to a signal generated by an optical sensor for detecting a reference element, normally a preprinted marker such as a bar code, repeated at predetermined intervals along the web.
In the case of processing units comprising a punch station, and two heat-seal stations for applying the patches and pull-off tabs respectively, a high degree of precision is required in positioning the web, especially at the tab seal station.
That is, to ensure perfect sealing of the holes in the web, the size of the patches and tabs must be proportional to the maximum offset between the work position of the heat-seal stations and the holes themselves. The size of the tabs, however, is a critical parameter, which directly determines the size of the frames and caps applied to the tabs, and which must therefore be kept as small as possible to avoid the obvious disadvantages in using relatively large caps.
Similarly, when the processing unit comprises, in addition to the punch station, a station for injection molding closable opening devices directly onto the respective holes in the web, the portion of the web surrounding each hole must be positioned correctly inside the molding cavity, to ensure correct flow of the thermoplastic material injected into the cavity and, hence, correct sealing of the edge of the hole on both sides of the web.
A demand therefore exists within the industry for even greater precision in processing the web at each station on the unit.